Emotient itself has been sold to advertisers to help assess viewer reactions to their ads as well as tested by doctors to interpret signs of pain among patients unable to express themselves, and a retailer used it to monitor shoppers’ facial expressions in store aisles, the company had said.

An Apple spokeswoman confirmed the purchase with the company’s standard statement after an acquisition, saying Apple “buys smaller technology companies from time to time, and we generally do not discuss our purpose or plans.” She declined to elaborate on the deal terms.

Emotient, based in San Diego, had previously raised $8 million from investors including Intel Capital. The company had been seeking a new round of venture-capital financing, but wasn’t able to secure it on favorable terms according to a person familiar with the matter.

The company this week revised its website and removed details about the services it had been selling.

Apple has expressed interest in the field. In a 2014 patent application, it described a software system that would analyze and identify people’s moods based on a variety of clues, including facial expression.

In October, Apple confirmed that it had acquired another artificial-intelligence startup, VocalIQ Ltd., that aims to improve a computer’s ability to understand natural speech.

In May, Emotient announced that it had been granted a patent for a method of collecting and labeling as many as 100,000 facial images a day so computers can better recognize different expressions.

It’s hard to say what’s coming down the line with this acquisition, but it’s still interesting.